linux-kernel - Re: [PATCH] ARM: tegra: Use PMC scratch register 40 for tegra

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Date:	Mon, 22 Dec 2014 20:27:55 +0300
From:	Dmitry Osipenko <digetx@...il.com>
To:	Stephen Warren <swarren@...dotorg.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Alexandre Courbot <gnurou@...il.com>,
	Joseph Lo <josephl@...dia.com>
CC:	stable@...r.kernel.org, Russell King <linux@....linux.org.uk>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Sekhar Nori <nsekhar@...com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Christoffer Dall <christoffer.dall@...aro.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: tegra: Use PMC scratch register 40 for tegra_resume()
 location store

22.12.2014 19:17, Stephen Warren пишет:
> On 12/21/2014 03:52 PM, Dmitry Osipenko wrote:
>> Commit 7232398abc6a ("ARM: tegra: Convert PMC to a driver") changed
>> tegra_resume()
>> location storing from late to early and as result broke suspend on tegra20.
>> PMC scratch register 41 was used by tegra lp1 suspend core code for storing
>> physical memory address of common resume function and in the same time used by
>> tegra20 cpuidle driver for storing cpu1 "resettable" status, so it implied
>> strict order of scratch register use. Fix it by using scratch 40 instead of 41
>> for tegra_resume() location store.
> 
> You likely can't simply change the PMC scratch register usage arbitrarily;
> specific registers are designated for specific purposes, and code outside the
> Linux kernel (bootloaders, LP0 resume code, secure monitors, etc.) may depend on
> those specific values being in those registers. Without significant research,
> I'd suggest not changing the PMC scratch register usage.

Sure, that's why I asked to verify if scratch register 40 is in use in the
comment after commit message. I've checked that u-boot doesn't use it (since
upstream kernel doesn't care about any other bootloader), but no idea about
secure monitor. It's definitely safer to avoid changing scratch regs usage, I
thought that proposed solution would be best from the pure code point of view.
So, I'm considering your answer as a rejection of the patch (please, let me know
if I'm wrong) and will prepare another one. Btw, it would be nice to have
scratch registers usage publicly documented somewhere (on "Tegra Public
Application Notes" webpage for example), if it's possible, of course.

-- 
Dmitry
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